Blood platelet analysis and quantification play an important role in many medical treatments. A microscope is typically used to take a picture of a fluid layer containing platelets. The fluid is sometimes covered by a transparent plate, usually a thin sheet of glass. In some applications no cover plate is used.
Analysis is performed to detect round shapes within a size range associated with platelets. Platelets appear dark in the image, This pattern is identified against a background of other objects such as red blood cell remnants and dust particles. The microscope focus is precisely determined in order to image the platelets as described above. The microscope objective lens is adapted to account for imperfect spherical aberration of the lens, and to account for the optical distortion produced the cover plate that covers the sample (if there is a cover plate at all).
White blood cells are cells found in the blood that form part of the body's defenses against foreign bodies. They are larger than platelets and red blood cells. There are 5 types of white blood cells of interest differ in size and shape. Altogether, there are approx. 7×109/liter white blood cells in blood.
Neutrophil granulocyte is the most common white blood cell to be found in the human blood (62%). They have a spherical shape when inactivated with a diameter of 10-12 μm and a segmented nucleus.
The Eosinophil granulocyte is less common (2.3%) and will be transparent if not dyed. Its shape is very similar to the neutrophil with its diameter also being around 10-12 μm. Eosinophils together with neutrophils and basophils are granulocytes meaning they have big granules their cytoplasm.
Basophil granulocyte is a white blood cell that is even less common (0.4%). As the name states it is very easily stained. If the cell is not dyed it will appear partly transparent as it is only the nucleus that is clearly visible.
Large granular lymphocytes have a diameter between 12-15 μm. The cell's nucleus occupies most of the cell and is clearly visible. The small granular Lymphocyte has the same shape however a diameter which is smaller (7-8 μm). The percentage of this white blood cell being in an adult's blood is approx. 30%.
Monocytes have a cell diameter between 7.72-9.99 μm and account for approx. 5% of the white blood cells in humans. Monocytes are very diverse in shape and thus can be easily confused for other cells. They can be recognized by their non-circular multi-, di-, tri-lobed nucleus.
Red blood cells are the most abundant cells in the blood stream. They have a disk diameter of approximately 6.2-8.2 μm and a thickness at the thickest point of 2-2.5 μm, with a minimum thickness in the center of 0.8-1 μm. They are bi concave in shape and a normal concentration for humans is approx. 5×106/μl.
The conventional microscopy approach to counting white blood cells and red blood cells in a sample is similar in many respects.